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What is a virus?
What is a virus?
• You’ve probably had the flu—influenza—at
some time during your life.
• Most biologists consider viruses to be
nonliving because they don’t exhibit all
the criteria for life.
• Nonliving particles called viruses
cause influenza.
• They don’t carry out respiration, grow, or
develop. All viruses can do is replicate—make
copies of themselves—and they can’t even do
that without the help of living cells.
• Viruses are composed of nucleic acids
enclosed in a protein coat and are smaller
than the smallest bacterium.
What is a virus?
What is a virus?
Capsid
Nucleic
acid
• However, sometimes scientists use code
numbers to distinguish among similar
viruses that infect the same host.
• A virus that infects a bacterium is called
a bacteriophage (bak TIHR ee uh fayj),
or phage for short.
Viral Structure
• A virus has an
inner core of
nucleic acid,
either RNA or
DNA, and an
outer protein coat
called a capsid.
Nucleic
acid
Envelope
• Envelopes are
composed
primarily of the
same materials
found in the
plasma
membranes of all
cells.
Viral Structure
Capsid
Nucleic
acid
Envelope
Envelope
Viral Structure
Capsid
• Other viruses were named for the organ or
tissue they infect.
• A cell in which a virus replicates is
called the host cell.
Viral Structure
• Today, most viruses are given a genus
name ending in the word “virus” and a
species name.
• Viruses, such as rabies viruses and
polioviruses, were named after the diseases
they cause.
• Viral nucleic acid is
either DNA or RNA and
contains instructions for
making copies of the
virus.
• Some relatively
large viruses, such
as human flu
viruses, may have
an additional
layer, called an
envelope,
surrounding their
capsids.
Viral Structure
Nucleic acid
Nucleic acid
• The tobacco mosaic
virus has a long,
narrow helical shape.
Capsid
Capsid
• Some viruses have only
four genes, while
others have hundreds.
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Viral Structure
Capsid
Nucleic acid
• The arrangement of
proteins in the
capsid of a virus
determines the
virus’s shape.
Viral Structure
Capsid
• Polyhedral viruses
resemble small
crystals.
Attachment to a host cell
Capsid
Nucleic
acid
Tail
Tail fiber
• A protein in the tail
fibers of the
bacteriophage T4
recognizes and
attaches the T4 to its
bacterial host cell.
Attachment to a host cell
Nucleic acid
• The protein
arrangement also
plays a role in
determining what
cell can be
infected and how
the virus infects
the cell.
Attachment to a host cell
Capsid
Nucleic
acid
Tail
• In other viruses, the
attachment protein is
in the capsid or in the
envelope.
Tail fiber
• Before a virus can replicate, it must
enter a host cell.
• A virus recognizes and attaches to a
host cell when one of its proteins
interlocks with a molecular shape that
is the receptor site on the host cell’s
plasma membrane.
Attachment is a specific process
• Each virus has a specifically shaped
attachment protein. Therefore, each
virus can usually attach to only a few
kinds of cells.
• In general, viruses are species specific,
and some also are cell-type specific. For
example, polio viruses normally infect
only intestinal and nerve cells.
Attachment is a specific process
Viral Replication Cycles
Viral Replication Cycles
• The species specific characteristic of
viruses is significant for controlling
the spread of viral diseases.
• Once attached to the plasma membrane
of the host cell, the virus enters the cell
and takes over its metabolism.
• The virus may inject its nucleic acid
into the host cell like a syringe injects
a vaccine into your arm.
• Only then can the virus replicate.
• The capsid of the virus stays attached
to the outside of the host cell.
• Viruses have two ways of getting
into host cells.
• An enveloped virus enters a host
cell in a different way.
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Viral Replication Cycles
Lytic cycle
• After attachment, the plasma membrane
of the host cell surrounds the virus and
produces a virus-filled vacuole inside
the host cell’s cytoplasm.
• Once inside the host cell, a virus’s
genes are expressed and the substances
that are produced take over the host
cell’s genetic material.
Lytic cycle
• Then, the virus bursts out of the
vacuole and releases its nucleic
acid into the cell.
• The viral genes alter the host cell
to make new viruses.
Bacteriophage
Nucleic
acid
Bacterial DNA
Bacterial
host cell
A. Attachment
B. Entry
The bacteriophage
injects its nucleic acid
into the bacterial cell.
E. Lysis and Release
The host cell breaks open and
releases new virus particles.
D. Assembly
New virus particles
are assembled.
C. Replication
The host’s metabolic
machinery makes viral
nucleic acid and proteins.
Lytic cycle
Lytic cycle
Lysogenic cycle
• The host cell uses its own enzymes, raw
materials, and energy to make copies of
viral genes that along with viral proteins
are assembled into new viruses, which
burst from the host cell, killing it.
• The new viruses
can then infect and
kill other host cells.
This process is
called a lytic (LIH
tik) cycle.
• Not all viruses kill the cells they infect.
Click image to play movie
• Some viruses go
through a lysogenic
cycle, a replication
cycle in which the
virus’s nucleic acid is
integrated into the host
cell’s chromosome.
Click image to play movie
Lysogenic cycle
Lysogenic cycle
Lysogenic cycle
• A lysongenic cycle begins in the same
way as a lytic cycle.
• Viral DNA that is integrated into the host
cell’s chromosomes is called a provirus.
• Therefore, every cell that originates
from an infected host cell has a copy
of the provirus.
• However, in a lysogenic cycle, instead of
immediately taking over the host’s genetic
material, the viral DNA is integrated into
the host cell’s chromosome.
• A provirus may not affect the functioning
of its host cell, which continues to carry
out its own metabolic activity.
• However, every time the host cell
reproduces, the provirus is replicated
along with the host cell’s chromosome.
• The lysogenic phase can continue for
many years. However, at any time, the
provirus can be activated and enter a
lytic cycle.
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Lysogenic cycle
B. Provirus Formation
A. Attachment and Entry Provirus
Bacterial host
chromosome
C. Cell Division
A lysogenic virus
The viral nucleic acid is called
injects its nucleic
a provirus when it becomes
acid into a bacterium. part of the host’s chromosome.
Although
the provirus
is inactive,
it replicates
along with
the host cell’s
chromosome.
LYSOGENIC CYCLE
LYTIC CYCLE The provirus leaves
the chromosome.
The cell breaks
open releasing Viral nucleic acid and
viruses.
proteins are made.
Disease symptoms of proviruses
Disease symptoms of proviruses
• Many disease-causing viruses have
lysogenic cycles.
• Another lysogenic
virus is the one that
causes chicken pox.
• Three examples of these viruses are
herpes simplex I, herpes simplex II that
causes genital herpes, and the hepatitis B
virus that causes hepatitis B.
Disease symptoms of proviruses
Disease symptoms of proviruses
Release of viruses
• Having chicken pox, which usually
occurs before age ten, gives lifelong
protection from another infection by
the virus. However, some chicken pox
viruses may remain as proviruses in
some of your body’s nerve cells.
• Later in your life, these proviruses
may enter a lytic cycle and cause a
disease called shingles—a painful
infection of some nerve cells.
• Either lysis, the bursting of a cell, or
exocytosis, the active transport process by
which materials are expelled from a cell,
release new viruses from the host cell.
Release of viruses
Retroviruses
Retroviruses
• In exocytosis, a newly produced virus
approaches the inner surface of the host
cell’s plasma membrane.
• Many viruses, such as the human
immunodeficiency virus (HIV) that causes
the disease AIDS, are RNA viruses—RNA
being their only nucleic acid.
• The RNA virus with the most complex
replication cycle is the retrovirus (reh tro VY
rus).
• The plasma membrane surrounds the virus,
enclosing it in a vacuole that then fuses
with the host cell’s plasma membrane.
HIV virus
HIV virus
• Then, the viruses are released
to the outside.
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Retroviruses
Retroviruses
• Once inside a host cell, the retrovirus
makes DNA from its RNA.
• This enzyme helps produce doublestranded DNA from the viral RNA.
• To do this, it uses reverse transcriptase,
an enzyme it carries inside its capsid.
• Then the double-stranded viral
DNA is integrated into the host
cell’s chromosome and becomes a
provirus.
Retroviruses
RNA
Retrovirus
RNA
Reverse
transcriptase
DNA
DNA is made from
the viral RNA.
Entering
cell
Provirus in
host chromosome
mRNA
Retrovirus Cycle
New virus parts
Exiting
cell
New virus
forming
HIV: An infection of white blood cells
HIV: An infection of white blood cells
HIV: An infection of white blood cells
• Once inside a
human host,
HIV infects
white blood
cells.
• Infected host cells still function
normally because the viral genetic
material is a provirus that produces
only a small number of new viruses
at a time.
• Most people with an HIV infection
eventually get AIDS because, over
time, more white blood cells are
infected and produce new viruses.
• Newly made viruses are released
into the blood stream by exocytosis
and infect other white blood cells.
• Because the infected cells are still able to
function normally, an infected person may
not appear sick, but they can still transmit
the virus in their body fluids.
• Because white blood cells are part of a
body’s disease-fighting system, their
destruction interferes with the body’s
ability to protect itself from organisms
that cause disease, a symptom of AIDS.
Cancer and Viruses
Prions and viroids
Prions and viroids
• Some viruses have been linked to certain
cancers in humans and animals.
• Researchers have recently discovered some
particles that behave somewhat like viruses
and cause infectious diseases.
• Prions are thought to act by causing other
proteins to fold themselves incorrectly,
resulting in improper functioning.
• These viruses disrupt the normal growth
and division of cells in a host, causing
abnormal growth and creating tumors.
• Prions are composed of proteins but have no
nucleic acid to carry genetic information.
• Prions are responsible for many animal
diseases, such as mad cow disease and its
human equivalent, Creutzfeldt-Jakob
disease.
Normal white blood cells
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Prions and viroids
Plant viruses
• Viroids are composed of a single circular
strand of RNA with no protein coat.
• The first virus to be identified was a
plant virus, called tobacco mosaic virus,
that causes disease in tobacco plants.
• Viroids have been shown to cause
infectious diseases in several plants.
Tobacco mosaic virus
causes yellow spots
on tobacco leaves,
making them
unmarketable.
• The amount of viroid RNA is much
less than the amount found in viruses.
Plant viruses
• Plant viruses require wounds or insect
bites to enter and infect a host, and do
not use surface recognition.
• They do not undergo lytic or
lysogenic phases.
Plant viruses
Plant viruses
• Not all viral plant diseases are fatal or
even harmful.
• Some mosaic viruses cause striking patterns
of color in the flowers of plants.
Rembrandt
tulips
• Viruses cause as many as 1000 plant
diseases and are named according to
their host plant.
• Viruses can cause stunted growth
and yield losses in their host plants.
Origin of Viruses
• For replication, viruses need host cells;
therefore, scientists suggest that viruses
might have originated from their host cells.
• Some scientists suggest that viruses are
nucleic acids that break free from their
host cells while maintaining an ability to
replicate parasitically within the host cells.
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